An amplifier is used in a communication system. Such an amplifier needs tuning in order to operate in good condition in multiple frequency bands. Conventionally, for the purpose of tuning an amplifier for amplifying a high-frequency signal, impedance matching is preformed by changing transconductance of a field effect transistor constituting the amplifier, or by using variable capacitors.
Such change of the transconductance of the field effect transistor or use of the variable capacitors, which performs impedance matching, changes gain of the amplifier to a large extent.
Japanese Patent Application Publication No. 2009-10826 discloses another technique to amplifier operate an amplifier in multiple frequency bands. According to the technique, the amplifier is provided with a degeneration portion including multiple inductors connected to multiple amplifying elements respectively.
The technique disclosed in the Patent Publication needs multiple high-frequency signal input terminals corresponding to the respective frequency bands, even though the multiple inductors of the degeneration portion are formed in a smallest possible occupation area. As a result, the technique can not achieve size reduction of the amplifier.